Electronic communication is a popular form of exchanging information between locations. When transmitting voice or data information it is generally desirable to maximize the amount of information within a particular frequency band and minimize the error rate. This is true of both wireless and non-wireless applications.
To most efficiently utilize available bandwidth, communication standards often specify maximum transmit power levels and minimum separation between channels. As a result, challenges exist during signal demodulation and signal recovery. Certain demodulation and signal recovery operations, while effective, consume power at a high rate. While power consumption is a relevant consideration in any type receiver, battery operated communication receivers are particularly sensitive to power usage issues. One example of a battery operation communication receiver is a receiver in a wireless telephone, such as for example a cellular telephone.
In addition to power usage constraints, the cost and performance of the receiver is also a consideration. By way of example, superheterodyne receivers are a widely adopted configuration for wireless communication receivers. However, as compared to a direct conversion or near zero IF (hereinafter referred to as direct conversion) receiver, superheterodyne receivers are undesirably expensive. As a result, direct conversion receivers offer performance at a lower cost per unit than superheterodyne receivers. A superheterodyne receiver mixes the RF signal down to an intermediate frequency (IF) and eventually down to baseband. In contrast to superheterodyne receivers, direct conversion receivers demodulate the received signal from the carrier frequency to the baseband without use or passage through an intermediate frequency (IF). By eliminating hardware required to process the signal through the intermediate frequency, the cost of the direct conversion receiver is reduced.
While direct conversion receivers of the prior art enjoy cost advantages over superheterodyne receivers, their use has drawbacks. One such drawback is a susceptibility to signal interference from unwanted extraneous signals, or jammers. Because the direct conversion receiver does not demodulate and filter at an intermediate frequency unwanted signals may follow a signal of interest to the baseband. These unwanted signals may interfere with signal reception, decoding, and processing thereby disrupting a user's ability to use the communication device or channel. In some instances a cellular telephone system may drop the call due to the interference from the unwanted signal. These interfering signals need not be on adjacent or alternate channels to cause interference to the demodulated desired signal. This is because one source of unwanted signal disruption is generated within the receiver's demodulator by means of second order distortion products in the down mixer (or demodulator) itself. Therefore any two or more signals in the band as a whole but spaced apart by a frequency equal to or less than the desired signal bandwidth may generate a second order interfering signal at baseband. Thus the ability to minimize second order products in the receive chain is of vital importance.
An additional drawback of the prior art was an inability to determine the cause of the poor reception. The poor reception may arise from a weak signal or an unwanted interfering signal being generated by second order products within the receiver.
Therefore, there is a need in the art for a communication receiver that adopts the benefits of a direct conversion receiver (i.e., lower cost, fewer components, and lower power consumption) yet reduces the undesirable effects of the unwanted signals that may demodulate into the baseband. There is also a need for a method and apparatus to aid in determining the reason for the poor reception.